具有指定提示的getaddrinfo(3) – > ai_socktype不返回IPv6地址

假设以下代码模仿resolveip实用程序的基本功能：

#define _POSIX_SOURCE     /* getaddrinfo() */
#include <sys/types.h>    /* getaddrinfo(), struct addrinfo, struct sockaddr */
#include <sys/socket.h>   /* getaddrinfo(), struct addrinfo, struct sockaddr, AF_* */
#include <netdb.h>        /* getaddrinfo(), struct addrinfo, struct sockaddr */
#include <arpa/inet.h>    /* inet_ntop() */
#include <stdio.h>        /* fprintf(), printf(), perror(), stderr */
#include <stdlib.h>       /* EXIT_SUCCESS */

int main(int argc, char** argv) {
  for(int i = 1; i < argc; ++i) { /* For each hostname */

    char* hostname = argv[i];    
    struct addrinfo* res;  /* We retrieve the addresses */
    if(getaddrinfo(hostname, NULL, NULL, &res) == -1) {
      perror("getaddrinfo");
      continue;
    }

    for(; res->ai_next; res = res->ai_next) { /* We print the addresses */
      switch(res->ai_addr->sa_family) {
        case AF_INET: {
          struct in_addr addr = ((struct sockaddr_in*)(res->ai_addr))->sin_addr;
          char buffer[17]; printf("%s: %s\n", hostname, inet_ntop(AF_INET, &addr, buffer, 17)); break;
        } case AF_INET6: {
          struct in6_addr addr = ((struct sockaddr_in6*)(res->ai_addr))->sin6_addr;
          char buffer[40]; printf("%s: %s\n", hostname, inet_ntop(AF_INET6, &addr, buffer, 40)); break;
        } default: {
          fprintf(stderr, "%s: Unknown address family\n", hostname);
        }
      }
    }

    freeaddrinfo(res); /* We release the allocated resources */

  } return EXIT_SUCCESS;
}

以google.com作为第一个也是唯一一个参数调用的上述代码将输出类似于：

google.com: 173.194.35.87
google.com: 173.194.35.87
google.com: 173.194.35.87
google.com: 173.194.35.95
google.com: 173.194.35.95
google.com: 173.194.35.95
google.com: 173.194.35.88
google.com: 173.194.35.88
google.com: 173.194.35.88
google.com: 2a00:1450:4008:800::101f
google.com: 2a00:1450:4008:800::101f

假设我们想要摆脱重复的条目.因此,让我们创建一个结构,其中包含有关我们希望检索哪种结果的提示.以下修改不会以任何方式影响输出,因为结构初始化为getaddrinfo(3)联机帮助页规定的默认值：

struct addrinfo hints = {
  .ai_family = AF_UNSPEC,
  .ai_socktype = 0,
  .ai_protocol = 0,
  .ai_flags = (AI_V4MAPPED | AI_ADDRCONFIG)
}; if(getaddrinfo(hostname, NULL, &hints, &res) == -1) {
  perror("getaddrinfo");
  continue;
}

现在让我们通过将ai_socktype字段指定为任意值来过滤掉重复的条目：

struct addrinfo hints = {
  .ai_family = AF_UNSPEC,
  .ai_socktype = SOCK_DGRAM,
  .ai_protocol = 0,
  .ai_flags = (AI_V4MAPPED | AI_ADDRCONFIG)
};

唉,我们现在丢失了IPv6地址：

google.com: 173.194.35.87
google.com: 173.194.35.95
google.com: 173.194.35.88

现在让我们回到原来的无提示版本：

if(getaddrinfo(hostname, NULL, NULL, &res) == -1) {
  perror("getaddrinfo");
  continue;
}

现在让我们使用手动过滤：

for(; res->ai_next; res = res->ai_next) {
  if(res->ai_socktype != SOCK_DGRAM) continue;
  ...
}

现在一切都按照预期的方式运行：

google.com: 173.194.35.87
google.com: 173.194.35.95
google.com: 173.194.35.88
google.com: 2a00:1450:4008:801::101f

我很好奇,从而提示传递给getaddrinfo(3)函数和手动过滤返回记录之间的差异.使用glibc 2.17在linux内核3.8.0-32上测试.

解决方法:

您的for循环检查是错误的,您总是跳过最后一个条目 – 在您的情况下恰好是IPv6地址.

 for(; res->ai_next; res = res->ai_next) {

需要是

 for(; res; res = res->ai_next) { 

标签：unix,c-3,linux,unix-socket,getaddrinfo
来源： https://codeday.me/bug/20190624/1281217.html